1. Field of the Invention
This invention relates to a compliant structure for reducing bending stress at the points of connection of an elongated cylindrical tether which may be used to connect a floating platform supported by a body of water to anchors in the floor thereof.
2. Description of the Prior Art
In recent years there has been considerable attention directed to the drilling and production of wells located in water, such as offshore in the Gulf of Mexico. Wells may be drilled in the ocean floor from either fixed platforms in relatively shallow water or from floating structures or vessels in deeper water. Fixed platforms usually consist of a working platform supported above the surface of the body of water by a plurality of piles or columns extending from the platform to the ocean floor. This works fairly well in shallow water, but as the water gets deeper the problems of design and accompanying costs become prohibitive. In deeper water it is common practice to drill from a floating vessel. However, the problem of producing the wells, from a permanent installation attached to the seafloor, remains.
In recent years there has been some attention directed toward many different kinds of floating structures. One kind which is attracting attention is the so-called vertically moored platform. A key feature of the vertically moored platform is that the floating platform is anchored to the sea floor only by elongated cylindrical tethers such as pipe. In a common form the floating platform includes a plurality, at least three, of elongated buoyant cylinders which are mutually supported in an upright position and are held to the ocean floor by elongated cylindrical tethers attached to seafloor anchor means.
However, it is possible to reduce the buoyancy means to a single vertical cylinder, and eliminate separate seafloor anchor bases by driving the tether into the marine bottom.
In order to limit vertical excursions of the buoyant single vertical cylinder, and to avoid stretching or buckling of the wells, excess buoyancy must be designed into the floating structure and the floating structure must be moored vertically and rigidly to the ocean floor. In such a vertically moored floating platform, or "Tension Leg Platform," wave induced excursions cause lateral surge, sway and yaw of the floating structure with resulting angular distortion of the vertical elongated cylindrical tethers. If the buoyancy means consists of a single elongated cylinder, or "sparbuoy", angular pitch and roll excursions add to the angular distortion in the elongated tether, particularly at point(s) of attachment to the pivoting body.
In such a vertically moored platform it is usually desirable to maintain the elongated tethers connecting the platform to the ocean floor under a substantial amount of tension. As these connecting tethers are under tension and are also subjected to an angular rotation caused by the excursion of the floating body, the influence of the tension tends to concentrate the maximum angular rotation of the tethers at their points of connection. The resultant maximum bending stresses are correspondingly concentrated at the ends of the tether(s). The resulting increased bending strains, however, should be maintained within acceptable limits.
It has also been proposed to utilize the elongated cylinder tether as a conduit for well conductors or flowlines and for transportation to or from the surface of the produced fluids. These functions make it desirable to fabricate a full open internal bore through the tether.
As disclosed in U.S. Pat. No. 3,559,410 issued Feb. 2, 1971, to Blenkarn et al., various apparatus are disclosed which attempt to reduce these stresses resulting from the bending in these hollow tubular connecting tethers. As shown in FIG. 4 of patent '410 a post is placed inside an elongated cylindrical tether adjacent its lower end to distribute a portion of the stresses from the elongated cylindrical tether to the post. This design, however, places a restriction in the elongated cylindrical tether to any fluid flowing or passing through well tubulars or flowlines located within or forming a portion of the elongated cylindrical tether, and maintenance or inspection costs associated with the operation of this device would be prohibitive since it is located in a relatively inaccessible location.
As described in U.S. Pat. No. 3,563,042 issued Feb. 16, 1971, to C. J. Ryan, a smooth trumpet-shaped cone may be used to limit the bending in cable-type tethers. However, such a device is vulnerable to marine fouling, corrosion, and wear, and is less adaptable to larger diameter cylindrical tethers which would require very exacting tolerances to avoid a contact stress problem.
A stress reduction connection device needs to be disclosed that is simple in design and is easy to install and maintain, and does not act as a flow restriction to fluids that may flow through an elongated cylindrical tether.